a) Field of the Invention
The present invention relates to an eyepiece system having a wide visual field which is to be used in combination with objective lens systems such as those of binoculars and so on.
b) Description of the Prior Art
As conventional examples of eyepiece systems which have wide visual fields, there are known eyepiece systems having apparent visual fields of 80.degree. which are disclosed by Japanese Patents Kokai Publication No. Hei 8-76032, No. Hei 8-36140, etc., an eyepiece system having an apparent visual field of 75.degree. which is disclosed by Japanese Patent Kokai Publication No. Hei 8-5938, an eyepiece system having an apparent visual field of 65.degree. which is disclosed by Japanese Patent Kokai Publication No. Hei 6-175047, and an eyepiece system having an apparent visual field of 72.degree. disclosed by Japanese Patent Kokai Publication No. Hei 6-109983.
When an attempt is made to configure an eyepiece system so as to have a wide visual field and a long eye relief (a distance on an optical axis as measured from a surface of the eyepiece system located on the eye side to an eye point thereof), the eyepiece system has a large outside diameter, thereby aggravating aberrations. Since images are harder to eclipse as an exit pupil diameter is larger, it becomes desirable that the eyepiece system, which has a wide field angle in particular, has a large exit pupil diameter for observing images which are not eclipsed even at marginal portions of a visual field.
Out of the conventional examples mentioned above, the eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-76032 has a wide apparent visual field of 80.degree. and an eye relief which is long as compared with a focal length of the eyepiece system as a whole. However, this eyepiece system as a whole has a short focal length, thereby narrowing an airspace to be reserved between an objective lens system and the eyepiece system. When this eyepiece system is combined with an objective lens system which has a large aperture for enlarging an exit pupil diameter, the eyepiece system hardly allows a sufficient optical axis length to be reserved for a prism, of a roof prism type in particular requiring a long optical path length, which is to be disposed between the objective lens system and the eyepiece system.
When the focal length of this conventional example is prolonged for obtaining a long prism length in this conventional example, the eye relief is prolonged with the apparent visual field kept unchanged, whereby the eyepiece system has an enlarged outside diameter and makes it difficult to reserve a minimum eye-to-eye width required for binoculars.
Further, the eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-36140 has the apparent visual field of 8.degree. and a long eye relief of 0.97 f (f represents a focal length of the eyepiece system as a whole). However, this eyepiece system has a large outside diameter due to a fact that a lens component disposed on the eye side in this eyepiece system is a cemented lens component which is composed of a negative lens element and a positive lens element, and the cemented lens component has a positive refractive power which is weak. Accordingly, this eyepiece system makes it difficult, like the eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-76032, to reserve the minimum eye-to-eye distance required for binoculars when its focal length is prolonged for reserving a space for disposing a prism.
The eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-5938 has the apparent visual field of 75.degree. and favorably corrects aberrations such as curvature of field. However, this eyepiece system also has a small exit pupil diameter and is obliged to be said as an insufficient eyepiece system. Further, this eyepiece system, like the eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-36140 mentioned above, uses a cemented lens component which consists of a negative lens element and a positive lens element on the eye side and has a weak refractive power, thereby having a large outside diameter.
The eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 6-175047 has an apparent visual field of 65.degree. and uses a first lens unit which is disposed on the object side of an intermediate image surface and has a strong refractive power, whereby this eyepiece system must use positive lens components having strong refracting powers for composing a second lens unit. When an attempt is made to further widen the apparent visual field, the positive lens components must be thickened for reserving required marginal thicknesses, thereby increasing a weight of the eyepiece system, disturbing images at marginal portions of a visual field and making it difficult to correct aberrations favorably.
Furthermore, the eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 6-109983 has an apparent visual field of 72.degree. and a short focal length like each of the conventional examples described above, and is therefore insufficient for reserving a large pupil diameter. When an attempt is made to prolong a focal length of this eyepiece system, an eye relief is prolonged with the apparent visual field kept unchanged, thereby enlarging an outside diameter of the eyepiece system.
The conventional eyepiece systems cannot have extremely large apparent visual fields and large pupil diameters as described above, though these eye piece systems correct aberrations favorably even at marginal portions of the visual fields.
Furthermore, a maximum diameter of an eyepiece system constitutes an important factor when the eyepiece system has an extremely wide visual field on the order of 80.degree..
FIG. 9 is a diagram schematically showing the relationship between the eye relief and diameter of an eyepiece system. When two eyepiece systems which have the same apparent visual field .alpha. are compared with each other, one which has a higher ratio of an eye relief ER relative to a maximum diameter DM of the eyepiece system (a product of an eye relief multiplied by an inverse number of a maximum diameter of the eyepiece system) can be said to be compact as seen from the drawing. In the drawing, the reference symbol E represents an eyepiece.
In addition to the eyepiece systems described above, an eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 7-84195 is a known conventional example of eyepiece system which has an extremely wide apparent visual field on the order of 80.degree.. Though this eyepiece system has an apparent visual field on the order of 80.degree., it has a ratio on the order of 0.37 between an eye relief and a maximum diameter of the eyepiece system, and cannot be said to be a sufficiently compact eyepiece system.
Moreover, an eyepiece system disclosed by Japanese Patent Kokai Publication No. Hei 8-36140 has a low ratio between an eye relief and a maximum diameter of the eyepiece system which is on the order of 0.3, and a small outside diameter when an eye relief is on the order of 11 mm as in an embodiment described in the bulletin of the patent. However, the eye relief of 11 mm is insufficient for favorable observation. When an attempt is made to prolong the eye relief, the eyepiece system has an outside diameter which is rather large. Further, it cannot be said that astigmatism and distortion are corrected sufficiently in this eyepiece system.
In addition, an eyepiece system disclosed by Japanese Patent Kokai Publication No. Sho 8-76032 has a high ratio between an eye relief and a maximum diameter of the eyepiece system which is on the order of 0.5, but cannot reserve a sufficient amount of marginal rays at an apparent visual field of 80.degree. due to insufficient marginal thicknesses (thicknesses of marginal portions of lens components). Further, this conventional example has an eye relief on the order of 15 mm on an optical axis, but uses a lens surface having high curvature on the eye side, thereby making a substantial eye relief shorter than 13 mm which is not sufficient.
As understood from the foregoing description, the conventional eyepiece systems which have super wide visual fields have apparent visual fields on the order of 80.degree., and can hardly correct curvature of field, astigmatism and distortion favorably while reserving a long eye relief which is substantially 15 mm or longer.